cmd/compile: fix Val vs Opt collision

Fixes #12686.

Change-Id: I7a9f49dbd1f60b1d0240de57787753b425f9548c
Reviewed-on: https://go-review.googlesource.com/17031Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/gc/const.go

@@ -1305,20 +1305,28 @@ func defaultlit(np **Node, t *Type) {
 	return
 
 num:
+	// Note: n.Val().Ctype() can be CTxxx (not a constant) here
+	// in the case of an untyped non-constant value, like 1<<i.
+	v1 := n.Val()
 	if t != nil {
 		if Isint[t.Etype] {
 			t1 = t
-			n.SetVal(toint(n.Val()))
+			v1 = toint(n.Val())
 		} else if Isfloat[t.Etype] {
 			t1 = t
-			n.SetVal(toflt(n.Val()))
+			v1 = toflt(n.Val())
 		} else if Iscomplex[t.Etype] {
 			t1 = t
-			n.SetVal(tocplx(n.Val()))
+			v1 = tocplx(n.Val())
+		}
+		if n.Val().Ctype() != CTxxx {
+			n.SetVal(v1)
 		}
 	}
 
-	overflow(n.Val(), t1)
+	if n.Val().Ctype() != CTxxx {
+		overflow(n.Val(), t1)
+	}
 	Convlit(np, t1)
 	lineno = int32(lno)
 	return

test/fixedbugs/issue12686.go

+// compile
+
+// Copyright 2015 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// golang.org/issue/12686.
+// interesting because it's a non-constant but ideal value
+// and we used to incorrectly attach a constant Val to the Node.
+
+package p
+
+func f(i uint) uint {
+	x := []uint{1 << i}
+	return x[0]
+}